1.1 Purpose of Investigation
This project is part of the Utah 1998 EPA-USGS AVIRIS study. The principal elements of this joint investigation include mapping and characterization of surficial minerals, watershed evaluation to better define effects of mineralization and related mining activities, and vegetation studies. These studies will also be used in support for the 2002 Olympics in Salt Lake City (see the USGS Utah Project web page).
1.2 Location and Significance of Study Area
The study area (Figure 1.1), located approximately 65 miles southwest of Salt Lake City in the southern part of the Oquirrh Mountains, includes the Camp Floyd (Mercur) mining district, Rush Valley, and the southeastern portion of the Tooele Army Depot. Mercur is the third largest gold producing district in Utah. Unlike the other major mining districts in the State which mine Gold as a by-product, the Mercur district is primarily a gold district. Silver and Mercury are mined as secondary metals (Koschmann and Bergendahl, 1968).
Bedded replacement deposits of gold and gold-mercury are the major ore deposits, and are found in the Great Blue Limestone. Fissure veins also contain replacement deposits of gold and gold-mercury, and silver and silver-lead. The gold-bearing replacement deposits contain pyrite (FeS2), realgar (AsS), orpiment (As2S3), and cinnabar (HgS) (Koschmann and Bergendahl, 1968). The Mercur Mine is listed as the principal occurrence for realgar and orpiment in the United States (Hurlbut and Klein, 1977).
1.3 Geologic Setting
The Oquirrh Mountains are composed of a Paleozoic sedimentary suite that is more that 22,000 feet thick. Along the western flank of the range (Mercur district), the primary units are the Mississippian Deseret Limestone, Humbug Formation, Great Blue Limestone, and Manning Canyon Shale. These units are deformed into a series of northwest trending folds (Figure 1.2), including the Ophir Anticline in the vicinity of the Mercur mine (Koschmann and Bergendahl, 1968). The folding is the result of Sevier style "thin skinned" deformation, with regional principal compression from the southwest (Atkinson, ed., 1976).
The sedimentary layers are intruded by various igneous dikes, and are
cut by numerous normal faults. The igneous dikes are predominantly composed
of quartz monzonite porphyry. South of the Mercur mine, stocks and sills
of Tertiary Eagle Hill rhyolite are also found (Gilluly, 1932).
2. METHODS OF INVESTIGATION
2.1 Data Acquisition, Calibration, and Processing
The AVIRIS data were acquired on August 5, 1998 with the NASA ER-2 at an altitude of 65,000 feet, and have a spatial resolution of approximately 17 meters/pixel. The radiance data were calibrated to surface reflectance using the standard U.S.G.S. Spectroscopy Lab methods (Clark et al, 1998). The calibration site for the Oquirrh region is located along the southern edge of Great Salt Lake at Old Saltair salt flat (40o 46.051' N, 112o 09.647' W) (Figure 1.1). Calibrated surface reflectance data were analyzed, and mapping results were produced using the U.S.G.S. Tetracorder algorithm (Clark et al, 1999).
The Tetracorder mapping results were grouped based on the presence of absorption features in specific regions of the spectra. The 1 micron group (minerals with absorption features in the 1 micron region) includes hematites, goethites, jarosite, and other Fe3+ bearing minerals. The 2 micron group includes the kaolinite group (well crystallized, poorly crystallized, and halloysite), carbonates (calcite and calcite mixtures), muscovites, and alunites. Vegetation was generally grouped as high chlorophyll content, moderate chlorophyll, dry + green vegetation, and dry vegetation.
3. SPECTRAL RESULTS AND MATERIAL MAPPING
3.1 Comparison of AVIRIS and Library Spectra
To verify minerals in the 1 micron and 2 micron groups, and different
vegetation types, comparative plots of spectra were produced to examine
the accuracy of the Tetracorder mapping results. Spectra were extracted
from the calibrated AVIRIS data, based on the mapped results, and the individual
absorption features compared to reference library spectra (see the U.S.G.S. Spectral Library). (Figures 3.1,
3.2 Material Mapping
Mineral and vegetation maps were produced, for the groupings as previously described, from the results of the Tetracorder algorithm. The image display program PicWorks (pw) was used to create overlay images of mineral groups and vegetation. The AVIRIS band 30 was used as a base gray scale image for spatial reference (Figure 3.4).
4.1 Spectral Analysis
Preliminary evaluation of the spectra of mapped materials indicates a reliable calibration for this data set. An important consideration in this evaluation is the mapping algorithms ability to differentiate materials with similar absorption features. This ability is largely dependent on the quality of the calibration method and implementation of the calibration procedure. In the case of the kaolinite group mineral spectra previously shown (Figures 3.1, 3.2, 3.3), detailed differentiation of crystal development in the mineral, and slight differences in related mineral species, can be achieved based on relative depth and width of doublet absorption features. The comparison between the AVIRIS spectra and the library spectra in the kaolinite group demonstrates a very good match in the 2 micron wavelength region. Similar results were observed in other spectral comparisons.
4.2 Interpretation and Analysis of Mapping
The most prominent features in the mosaicked mineral maps (Figure 3.4) are the alluvial fan (location A) and exposed deposits of the Mercur Mine. The image suggests that the source material (kaolinite group clays in the figure shown) for the alluvial fan originated in the mine site. Another exposure of kaolinite can be seen (location B) between the fan deposits and the mine, and may have contributed to the outwash material.
A curious feature of the image is that there appears to be no deposition of kaolinite in Mercur Canyon between the mine and the alluvial fan. This suggests that high velocity flow through the canyon may have prevented deposition of finer grained materials, which could not be deposited until they reached the lower gradient of the valley. It is also possible that deposition of kaolinite clays did occur in the canyon, but were not detected because of vegetation cover or shading in the narrow canyon.
Along the southwest flank of the Oquirrh range carbonates and muscovites mapped as expected, given the geologic setting described earlier. Mineral exposures are not seen at higher elevations (northeast part of the image) due to increased vegetation cover.
4.3 Field Verification and Further Investigation
Field verification and investigation for this study area is scheduled to begin in the summer of 1999. The field work for this area will focus on characterizing the alluvial fan deposits. Important considerations will include collection and analysis of the clays which might be important in transport of heavy metals from historical mining operations or natural occurrences. Elevated levels of arsenic and mercury (as high as 3300 mg/kg and 14 mg/kg respectively) have been reported in the outwash deposits (Bureau of Land Management, 1989).
The AVIRIS data provides an excellent means of determining the extent
of the alluvial fan deposits based on detailed mapping of the mineralogy.
On the ground, these types of deposits can appear geologically and visually
uniform. Traditional remote sensing imagery (i.e. aerial photography),
even with higher spatial resolution, may not be able to characterize subtle
differences in materials. This project illustrates how imaging spectroscopy
can be used to develop an integrated strategy for geologic and environmental
Atkinson, W.W., editor, Geology of the Oquirrh Mountains and Regional Setting of the Bingham Mining District, Utah, Utah Geological Association, Publication 6, 1976.
Clark, R.N., G.A. Swayze, K., and T.V.V. King, Imaging Spectroscopy: A Tool for Earth and Planetary System Science Remote Sensing with the USGS Tetracorder Algorithm, to be submitted to Science as a research article, in USGS Review, 1998. (Status: the paper, delayed from 1997 over Tricorder licensing, is in USGS review.)
Clark, R.N., G.A. Swayze, T.V.V. King, K.E. Livo, R.F. Kokaly, J.B. Dalton, J.S. Vance, B.W. Rockwell, R. R. McDougal, Surface Reflectance Calibration of Terrestrial Imaging Spectroscopy Data: a Tutorial Using AVIRIS. U.S. Geological Survey, Open File Report, 1998.
Gilluly, J., Geology and ore deposits of the Stockton and Fairfield quadrangles, Utah, USGS Professional Paper 173, 1932.
Hurlbut, C.S. Jr., C. Klein, Manual of Mineralogy, 19th edition. New York: John Wiley & Sons, 1977.
Koschmann, A.H., M.H. Bergendahl, Principal Gold-Producing Districts of the United States, USGS Professional Paper 610, 1968.
Tooker, E.W., R.J. Roberts, Upper Paleozoic Rocks in the Oquirrh Mountains and Bingham Mining District, Utah, USGS Professional Paper 629-A, 1970.
United States Bureau of Land Management, Region VIII, Site Investigation, Mercur Canyon outwash site investigation report executive summary, prepared by ICF Technology incorporated, Lakewood, CO, 1989.
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